Soft resins such as butadiene rubber (BR), isoprene rubber (IR) and styrene-butadiene rubber (SBR) have been hitherto used for molded articles such as tires. However, if the soft resins are used molded articles requiring weathering resistance, heat resistance and low-temperature resistance (e.g., various automobile parts), the resins tend to undergo oxidative deterioration thereby to be lowered in the properties. Therefore, further improvement of the weathering resistance, the heat resistance and the low-temperature resistance of the resins is eagerly desired.
The reason why lowering of properties is brought about when molded articles of the soft resins are used under severe conditions is that the double bonds in the main chains of the resins give rise to oxidative deterioration. Therefore, it is desired to decrease the quantity of the double bonds in the main chain to the smallest.
When the soft resins such as SBR, EPR and ethylene/propylene/nonconjugated diene copolymer (EPT) are used for rubber molded articles, they are subjected to peroxide crosslinking or radical modification. Therefore, elastomers having high crosslinking efficiency and high reaction efficiency, being almost free from lowering of properties after reaction and being inexpensive are desired.
Especially in case of peroxide crosslinking, a large amount of a peroxide is necessary if the crosslinking efficiency of the soft resin is low. Use of a large amount of a peroxide occasionally causes decomposition reaction of the main chain of the soft resin. Additionally, use of a large amount of a peroxide may cause a problem of remaining of the peroxide in the resulting crosslinked product.
The conventional EPT is still insufficient in the crosslinking efficiency, and especially when it is subjected to peroxide crosslinking, the properties of the resulting crosslinked product are not always satisfactory. Therefore, further improvement of the properties such as compression set is desired.
The present inventors have earnestly studied to solve such problems as mentioned above. As a result, they have found that an unsaturated copolymer, which is obtained by random copolymerizing an .alpha.-olefin, a specific conjugated diene monomer and optionally an aromatic vinyl compound and which has such properties that: the content of 1,2-addition units (including 3,4-addition units) derived from the diene is high, double bonds are present in the main chain and the side chain of the copolymer in a specific ratio, five-membered rings are present in the copolymer, and the double bonds and the five-membered rings are present in a specific ratio, is excellent in compatibility, weathering resistance, heat resistance, low-temperature resistance, crosslinking efficiency and modification efficiency. The present inventors have also found that the above copolymer has a low glass transition temperature and is capable of being industrially produced efficiently. Based on the finding, the present invention has been accomplished.
With respect to an ethylene/1,3-butadiene copolymer, the following literature (1) to (5) is known.
(1) Journal of Polymer Science, Part B, Polymer Physics, Vol. 26, 2113-2126 (1988), discloses an ethylene/1,3-butadiene copolymer prepared by the use of a supported Ziegler catalyst. According to this literature, the copolymer has butadiene-butadiene bond and is a block-like copolymer. The activity of the catalyst is low (not more than 10 kg/g-Ti). The butadiene units are mainly trans-1,4-addition units, and methyl methacrylate (MMA) or maleic anhydride is grafted on the double bond positions of the addition units to prepare a graft polymer with MMA.
(2) Makromol. Chem. 179, 2173-2185 (1978), discloses an ethylene/1,3-butadiene copolymer prepared by the use of a vanadium catalyst. According to this literature, 1,2-addition unit from the 1,3-butadiene is hardly contained in the copolymer, and the butadiene units are mainly trans 1,4-addition units. Besides, activity of the catalyst is low.
(3) Polymer Bulletin 31, 271-278 (1993), discloses an ethylene/propylene/1,3-butadiene copolymer prepared by the use of Ziegler catalyst. According to this literature, 1,2-addition units and 1,4-addition units from the 1,3-butadiene and propylene units are confirmed to be present in the copolymer, but the activity of the catalyst is low (not more than 10 kg/g-Ti).
The catalysts described in the literature (1) to (3) are low in the activity, and the resulting copolymers do not have any five-membered ring and any three-membered ring. The copolymers described in the literature (1) and (2) do not have any 1,2-addition unit derived from the butadiene.
(4) Makromol. Chem. 192, 2591-2601 (1991), discloses an ethylene/1,3-butadiene copolymer and an ethylene/propylene/1,3-butadiene copolymer both prepared by the use of a metallocene catalyst of biscyclopentadienyl type. According to this literature, the copolymers do not have any 1,2-addition unit and any three-membered ring though they have 1,4-addition units and five-membered rings. Besides, the activity of each catalyst is low (5 kg/mM-Zr).
(5) International Publication WO88/04672 (applied in 1986; corresponding to U.S. Pat. No. 5,191,052 applied on Mar. 2, 1993; corresponding to European Patent No. EP0275676B1 applied on Sep. 7, 1994), discloses an ethylene/1,3-butadiene copolymer prepared by the use of a metallocene catalyst of biscyclopentadienyl type. According to this publication, the copolymer has 1,2-addition units, 1,4-addition units and five-membered rings, but the quantity ratio therebetween is not described. In the examples, the quantity of the 1,2-addition units which are particularly useful is very small, and any three-membered ring structure is not referred to. Besides, the activity of the catalyst is very low (not more than 1 kg/mM-Zr.multidot.hr).